z~7-10 Galaxies in the HUDF and GOODS fields: UV Luminosity Functions
Bouwens, Rychard J.; Illingworth, Garth D.; Franx, Marijn; Ford, Holland
Abstract
We use all available deep optical ACS and near-IR data over both the HUDF and the two GOODS fields to search for star-forming galaxies at z>~7 and constrain the UV LF within the first 700 Myr. Our data set includes ~23 arcmin2 of deep NICMOS J+H data and ~248 arcmin2 of ground-based (ISAAC+MOIRCS) data, coincident with ACS optical data of greater or equal depths. In total, we find eight ~7.3 z-dropouts in our search fields, but no z~9 J-dropout candidates. A careful consideration of a wide variety of different contaminants suggest an overall contamination level of just ~12% for our z-dropout selection. After performing detailed simulations to accurately estimate the selection volumes, we derive constraints on the UV LFs at z~7 and z~9. For a faint-end slope α=-1.74, our most likely values for M*UV and φ* at z~7 are -19.8+/-0.4 mag and 1.1+1.7-0.7×10-3 Mpc-3, respectively. Our search results for z~9 J-dropouts set a 1 σ lower limit on M*UV of -19.6 mag assuming that φ* and α are the same as their values at slightly later times. This lower limit on M*UV is 1.4 mag fainter than our best-fit value at z~4, suggesting that the UV LF has undergone substantial evolution over this time period. No evolution is ruled out at 99% confidence from z~7 to z~6 and at 80% confidence from z~9 to z~7. We find that the mass-to-light ratio of halos evolves as ~(1+z)-1 if we require that the observed brightening in M*UV with redshift [i.e., M*UV=(-21.02+/-0.09)+(0.36+/-0.08)(z-3.8)] be consistent with the expected evolution in the halo mass function. Finally, we consider the shape of the UV LF at z>~5 and discuss the implications of the Schechter-like form of the observed LFs, particularly the unexpected abrupt cutoff at the bright end.